As a conventional method for extracting alkali metal and/or alkali earth metal from a solid containing the alkali metal and/or alkali earth metal, there is known e.g. a method disclosed in Patent Document 1 identified below. Patent Document 1 discloses a method for extracting magnesium and calcium from iron steel slag or the like, in which method the steel slag or the like is added to an aqueous solution containing formic acid or citric acid to allow elution of magnesium and calcium therein; then, carbon dioxide gas is infused into the aqueous solution to allow precipitation thereof as carbonates (magnesium carbonate and calcium carbonate).